This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.

The recvfrom() function shall receive a message from a connection-
mode or connectionless-mode socket. It is normally used with
connectionless-mode sockets because it permits the application to
retrieve the source address of received data.
The recvfrom() function takes the following arguments:
socket Specifies the socket file descriptor.
buffer Points to the buffer where the message should be stored.
length Specifies the length in bytes of the buffer pointed to by
the buffer argument.
flags Specifies the type of message reception. Values of this
argument are formed by logically OR'ing zero or more of
the following values:
MSG_PEEK Peeks at an incoming message. The data is
treated as unread and the next recvfrom() or
similar function shall still return this
data.
MSG_OOB Requests out-of-band data. The significance
and semantics of out-of-band data are
protocol-specific.
MSG_WAITALL On SOCK_STREAM sockets this requests that the
function block until the full amount of data
can be returned. The function may return the
smaller amount of data if the socket is a
message-based socket, if a signal is caught,
if the connection is terminated, if MSG_PEEK
was specified, or if an error is pending for
the socket.
address A null pointer, or points to a sockaddr structure in
which the sending address is to be stored. The length and
format of the address depend on the address family of the
socket.
address_len Either a null pointer, if address is a null pointer, or a
pointer to a socklen_t object which on input specifies
the length of the supplied sockaddr structure, and on
output specifies the length of the stored address.
The recvfrom() function shall return the length of the message
written to the buffer pointed to by the buffer argument. For message-
based sockets, such as SOCK_RAW, SOCK_DGRAM, and SOCK_SEQPACKET, the
entire message shall be read in a single operation. If a message is
too long to fit in the supplied buffer, and MSG_PEEK is not set in
the flags argument, the excess bytes shall be discarded. For stream-
based sockets, such as SOCK_STREAM, message boundaries shall be
ignored. In this case, data shall be returned to the user as soon as
it becomes available, and no data shall be discarded.
If the MSG_WAITALL flag is not set, data shall be returned only up to
the end of the first message.
Not all protocols provide the source address for messages. If the
address argument is not a null pointer and the protocol provides the
source address of messages, the source address of the received
message shall be stored in the sockaddr structure pointed to by the
address argument, and the length of this address shall be stored in
the object pointed to by the address_len argument.
If the actual length of the address is greater than the length of the
supplied sockaddr structure, the stored address shall be truncated.
If the address argument is not a null pointer and the protocol does
not provide the source address of messages, the value stored in the
object pointed to by address is unspecified.
If no messages are available at the socket and O_NONBLOCK is not set
on the socket's file descriptor, recvfrom() shall block until a
message arrives. If no messages are available at the socket and
O_NONBLOCK is set on the socket's file descriptor, recvfrom() shall
fail and set errno to [EAGAIN] or [EWOULDBLOCK].

Upon successful completion, recvfrom() shall return the length of the
message in bytes. If no messages are available to be received and the
peer has performed an orderly shutdown, recvfrom() shall return 0.
Otherwise, the function shall return −1 and set errno to indicate the
error.

The recvfrom() function shall fail if:
EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and no data
is waiting to be received; or MSG_OOB is set and no out-of-
band data is available and either the socket's file descriptor
is marked O_NONBLOCK or the socket does not support blocking
to await out-of-band data.
EBADF The socket argument is not a valid file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR A signal interrupted recvfrom() before any data was available.
EINVAL The MSG_OOB flag is set and no out-of-band data is available.
ENOTCONN
A receive is attempted on a connection-mode socket that is not
connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The specified flags are not supported for this socket type.
ETIMEDOUT
The connection timed out during connection establishment, or
due to a transmission timeout on active connection.
The recvfrom() function may fail if:
EIO An I/O error occurred while reading from or writing to the
file system.
ENOBUFS
Insufficient resources were available in the system to perform
the operation.
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 RECVFROM(3P)